1. Field of the Invention
The invention relates to a screen for producing fluids from subsurface wells and, more particularly, to a well screen having perforations, passages or slots packed with, and the interior and exterior surfaces coated with, an impermeable inorganic matrix. The invention further relates to methods for producing fluids from wells utilizing the screen coated with the impermeable inorganic matrix.
2. Description of the Prior Art
In the completion of wells that produce oil, water, or other fluids, production of fluids from loosely consolidated formations containing sand, silt, or clay presents substantial problems. Such formations contain little or no naturally-occuring binding or cementing materials. As a result, normal fluid flow conditions tend to cause particles from the formations to run into the well bore with attendant deteriorative effects upon fluid production from the well. Clogging of well screens and damage to pumping equipment are examples of problems resulting from sand flow. For the foregoing reasons, proper well completion for production of fluids from loosely consolidated formations requires use of some means to prevent entry of any substantial quantities of formation particles into the well liner. Accordingly, control of sand flow in wells is commonly provided by a sand or gravel pack placed exteriorly of the well liner. One practice is to pack the space between the well liner and the unconsolidated formation with sand or gravel after the liner is set in place. Another practice is to use a pre-filled liner in which sand or gravel is packed within the annular space formed between inner and outer liners.
In the use of sand or gravel packs, control of sand flow is accomplished because the majority of the formation particles are entrapped as they are carried by produced fluids through the interstices of the pack. This occurs as a result of "bridging," by which phenomenon a stable structure of solid particles is built within an opening several times the diameter of the particles that might be expected to flow through the opening.
In many of these procedures of gravel packing a subterranean well, it has been desirable to temporarily close the openings or perforations through the screen in order to prevent contaminant from entering through the perforations within the screen while the liner is being placed within the bore of the well, such that the contaminant, especially particulate contaminant, will stabilize within the perforation itself, thus clogging the screen and rendering it ineffective or less effective for subsequent treatment and production of the well.
Exemplary of prior art compositions for coating screens are those in U.S. Pat. No. 3,216,497, which discloses utilization of an aqueous slurry, paraffin wax, or an oil-soluble hydrocarbon resin. Additionally, this patent discloses, for use in situations where the screen will be contacted with oil, a coating of a water soluble material such as sugar, starch, polyvinyl alcohol, sodium chloride, or other similar compounds which can be crystalized on the screen, then subsequently dissolved with water. This patent also discloses utilization of a coating of a thin sheet of a metallic substance, such as magnesium, which is subsequently dissolved with hydrochloric acid. Polymers such as polyvinyl chloride or polyvinyl acrylate also may be utilized. These organic polymers are subsequently dissolved with tetrachloroethylene.
U.S. Pat. No. 3,273,641 teaches the utilization of plugs of various forms preferably made from a fusible metal or metallic alloy having a melting point which is above the normal reservoir temperature or hydration temperature of cement. These plugs, which are contained within the interior of the perforated or slotted liner, have a melting point which is low enough that a heated fluid, such as steam, hot water, or hot oil can be injected into the interior of the liner or circulated exteriorly thereof to melt the plugs. The fusible alloys used in the composition of the plugs generally are eutectic mixtures of bismuth, tin, lead, cadmium, indium, and antimony in various compositions, combinations and percentages.
U.S. Pat. No. 3,322,199 teaches the use of certain wax materials to reduce permeability and flow to groups of slots filling the well liner with a fluid at a temperature to sufficiently melt the waxy substance in a given group of slots. Preferable, the waxes will either be animal or vegetable waxes because of the property of slight or no solubility in crude oils. For example, animal waxes, such as crude grades or stearic acid, and vegetable waxes, such as carnauba, Japan, and Candelilla waxes may be utilized. Additionally, various synthetic paraffins, such as low molecular weight polyethylenes may be utilized. Gradation of melting points may be achieved by mixtures of the waxes of different melting points or by initial fractionation by means of solvents.
U.S. Pat. No. 3,333,635 discloses the use of a filler material within the bore of the liner which can be easily removed by utilization of a mild solvent. For example, sulphur, heavy grease, and very low melting point metallic alloys which can be removed by hot water may be utilized. Additionally, eutectic mixtures of bismuth, tin, cadmium, indium and antimony may be utilized.
U.S. Pat. No. 3,880,233 teaches the use of certain fatty alcohols, thermal plastic resins and waxes as plugging agents for perforated or slotted liners.
U.S. Pat. No. 3,905,423 discloses plugging and coating materials such as hydrated nitrates of chromium, iron, mercury and nickel. Additionally, this patent discloses plugging agents of pure organic compounds, such as acids, paraffins, gilsonite, beeswax, and certain metals and metallic alloys. Additionally, the patent discloses the utilization of a blend of a wax and a polymer for a plugging agent.
It has been found that these prior art waxes and organic polymer materials may dissolve at too low a temperature when exposed to fluid within the well bore while the screen is being placed immediate perforations in the well. Accordingly, these prior art materials are useful only at the given softening point or melting point of the sealant.
The present invention obviates the problems associated with utilization of prior art compositions by providing a coating composition which may be readily and easily applied completely or partially to the exterior of the screen prior to insertion of the screen into the well bore. The composition is readily available, inexpensive to obtain and prepare, is easily applicable to the exterior of the screen, and has been found to be extremely durable, yet is easily dissolved by utilization of a strong acid or an acid material having a highly disassociatable hydrogen ion. Additionally, utilization of the present composition as a coating for the liner screen provides a screen which is substantially sealed and will prevent penetration therethrough of particulate contaminant when the liner is inserted within the bore of the well and eliminates the need for subsequent utilization of a wash pipe which would be carried within the interior of the screen. The composition protects the screen and the passageways or slots therein from damage and plugging during shipping and handling and while the screen is being run into the well.